Pipelines are used for transmitting petroleum, natural gas, and the like. Such pipelines are made up of a plurality of line pipes. For the line pipes, welded steel pipes represented by a UOE steel pipe are used, for example.
A welded steel pipe is produced by, for example, the following method. Widthwise end portions of a steel plate are bent by a C-press (C-forming). The C-formed steel plate is bent by a U-press (U-forming). The U-formed steel plate is bent by an O-press (O-forming). As a result, a substantially circular open pipe is obtained in which the widthwise end portions of the steel plate are opposed to each other. In the open pipe, the widthwise end portions opposing each other in the circumferential direction are tack welded. Thereafter, inner surface welding and outer surface welding are performed on the open pipe. Through the processes described above, a welded pipe is obtained. In order to improve the roundness of the welded pipe, the welded pipe is expanded by a pipe expanding machine. As a result, a targeted welded steel pipe (a UOE steel pipe in this example) is produced.
An example of the pipe expanding machine is disclosed in, for example, JP2006-28439A. The pipe expanding machine includes a pipe expanding head. The pipe expanding head expands the welded pipe over its entire length while moving in the axial direction of the welded pipe relative to the welded pipe.